Awd vehicle with disconnect system

ABSTRACT

A vehicle drive train for transferring torque to first and second sets of wheels includes a first driveline adapted to transfer torque to the first set of wheels and a synchronizing clutch. A second driveline is adapted to transfer torque to the second set of wheels and includes a power disconnection device and a friction clutch. A hypoid gearset is positioned within the second driveline in a power path between the synchronizing clutch and the power disconnection device. The friction clutch and the power disconnection device are positioned on opposite sides of the hypoid gearset. The hypoid gearset is selectively disconnected from being driven by the first driveline, the second driveline or the wheels when the synchronizing clutch and the power disconnection device are operated in disconnected, non-torque transferring, modes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 13/144,439 filedJul. 13, 2011 which is a U.S. national stage of InternationalApplication No. PCT/US2010/021572 filed Jan. 21, 2010 which claims thebenefit of U.S. Provisional Ser. No. 61/145,985 filed Jan. 21, 2009. Theentire disclosure of each of the above-noted applications isincorporated herein by reference.

FIELD

The present disclosure relates to a driveline for a motor vehicle havinga system for disconnecting a hypoid ring gear from rotating at drivelinespeed. In particular, a power take-off unit includes a coupling forceasing the transfer of torque from a power source to a rear drivelinewhile another disconnect selectively interrupts the flow of power from avehicle wheel to a hypoid ring gear of the rear driveline. A torquecoupling selectively connects a portion of rear driveline with an inputto the hypoid ring gear.

BACKGROUND

Typical power take-off units transfer power from a transaxle in receiptof torque from a vehicle power source. The power take-off unit transferspower to a propeller shaft through a gear arrangement that typicallyincludes a hypoid cross-axis gearset. Other gear arrangements such asparallel axis gears may be provided within the power take-off unit toprovide additional torque reduction.

Power take-off units have traditionally been connected to the transaxleoutput differential. Accordingly, at least some of the components of thepower take-off unit rotate at the transaxle differential output speed.Power losses occur through the hypoid gear churning through alubricating fluid. Efficiency losses due to bearing preload and gearmesh conditions are also incurred while the components of the powertake-off unit are rotated.

Similar energy losses occur when other driveline components are rotated.For example, many rear driven axles include hypoid gearsets having aring gear at least partially immersed in a lubricating fluid. In atleast some full-time all-wheel drive configurations, the rear drive axlehypoid gearset continuously rotates during all modes of operation andtransmits a certain level of torque. In other applications, the rearaxle hypoid gearset still rotates but with out the transmission oftorque whenever the vehicle is moving. Regardless of the particularconfiguration, churning losses convert energy that could have beentransferred to the wheels into heat energy that is not beneficiallycaptured by the vehicle. As such, an opportunity may exist to provide amore energy efficient vehicle driveline.

SUMMARY

A vehicle drive train for transferring torque to first and second setsof wheels includes a first driveline adapted to transfer torque to thefirst set of wheels and a synchronizing clutch. A second driveline isadapted to transfer torque to the second set of wheels and includes apower disconnection device and a friction clutch. A hypoid gearset ispositioned within the second driveline in a power path between thesynchronizing clutch and the power disconnection device. The frictionclutch and the power disconnection device are positioned on oppositesides of the hypoid gearset. The hypoid gearset is selectivelydisconnected from being driven by the first driveline, the seconddriveline or the wheels when the synchronizing clutch and the powerdisconnection device are operated in disconnected, non-torquetransferring, modes.

Furthermore, a vehicle drive train for transferring torque from a powersource to first and second sets of wheels includes a first drivelineadapted to transfer torque from the power source to the first set ofwheels and includes a power take-off unit. The first driveline includesa differential, a first hypoid gearset and a synchronizer positionedbetween the differential and the first hypoid gearset to selectivelytransfer or cease the transfer of torque from the power source to thefirst hypoid gearset. A second driveline is in receipt of torque fromthe first hypoid gearset and transfers torque to the second set ofwheels. The second driveline includes a power disconnection deviceselectively interrupting the transfer of torque from the second set ofwheels to the first hypoid gearset. The second driveline also includes afriction clutch for transferring torque between the first hypoid gearsetand a second hypoid gearset associated with the second driveline.

Furthermore, a method for transferring torque from a power source to afirst pair and a second pair of wheels in a vehicle drive train isdisclosed. The method includes transferring torque from the power sourceto the first pair of wheels through a first transmission device. Asynchronizing clutch, within the first power transmission device, isactuated to transfer torque to a driveline interconnecting the firstpair and second pair of wheels. A friction clutch is subsequentlyactuated to transfer torque from the driveline to a rear drive axle toinitiate rotation of a gearset within the rear drive axle. The methodfurther includes actuating a disconnect to drivingly interconnect ashaft coupled to one wheel of the second pair of wheels and a rotatablemember of the rear drive axle once speed synchronization between thecomponents coupled by the disconnect is achieved.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a schematic of an exemplary vehicle equipped with a vehicledrive train of the present disclosure;

FIG. 2 is an enlarged schematic depicting a portion of the drive trainshown in FIG. 1;

FIG. 3 is an enlarged schematic depicting another portion of the drivetrain shown in FIG. 1;

FIG. 3A is an enlarged schematic depicting an alternate portion of thedrive train shown in FIG. 1;

FIG. 4 is a schematic of another exemplary vehicle equipped with anotheralternate drive train; and

FIG. 5 is an enlarged schematic depicting a portion of the drive traindepicted in FIG. 4.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

In general, the present disclosure relates to a coupling and hypoiddisconnect system for a driveline of a motor vehicle. A power take-offunit may be equipped with a synchronizer to disconnect the power sourcefrom a portion of the driveline and to reconnect through synchronizationof the driveline. A dog or roller-type clutch may be provided todisconnect a portion of the driveline from one or more of the vehiclewheels. Additionally, a friction coupling may be positioned in serieswithin the driveline to provide speed synchronization between front andrear driveline components when a power reconnection is desired. Thehypoid gearing of the vehicle driveline may be separated from thedriving source of power to reduce churning losses and other mechanicalinefficiencies.

With particular reference to FIGS. 1-3 of the drawings, a drive train 10of a four-wheel drive vehicle is shown. Drive train 10 includes a frontdriveline 12 and a rear driveline 14 both drivable from a source ofpower, such as an engine 16 through a transmission 18 which may be ofeither the manual or automatic type. In the particular embodiment shown,drive train 10 is a four-wheel system incorporating a power transmissiondevice 20 for transmitting drive torque from engine 16 and transmission18 to front driveline 12 and rear driveline 14. Power transmissiondevice 20 is shown as a power take-off unit.

Front driveline 12 is shown to include a pair of front wheels 24individually driven by a first front axle shaft 26 and a second frontaxle shaft 28, as well as a differential assembly 32. Power take-offunit 20 includes a reduction speed gearset 30, a synchronizer clutch 34,an output gearset 35 and a right-angled drive assembly 36.

Rear driveline 14 includes a propeller shaft 38 connected at a first endto right-angled drive assembly 36 and at an opposite end to one side ofa friction coupling 39. The opposite side of friction coupling 39 isconnected to a rear axle assembly 40. Rear driveline 14 also includes apair of rear wheels 42 individually driven by a first rear axle shaft 44and a second rear axle shaft 46. Rear axle assembly 40 also includes ahypoid ring and pinion gearset 48 driving a differential assembly 50. Adisconnect 52 selectively drivingly disconnects second rear axle shaft46 from ring and pinion gearset 48 and differential assembly 50.

Reduction speed gearset 30 of power take-off unit 20 includes a drivegear 56 fixed for rotation with an output shaft of transmission 18. Adriven gear 58 is in constant meshed engagement with drive gear 56 andis also fixed for rotation with a carrier 60 of differential assembly32. Differential assembly 32 includes a first side gear 62 fixed forrotation with first front axle shaft 26 and a second side gear 64 fixedfor rotation with second front axle shaft 28. Each of first and secondside gears 62, 64 are in meshed engagement with pinion gears 66, 68which are rotatably supported by carrier 60.

Power take-off unit 20 also includes an input shaft 76 supported forrotation within a housing. Input shaft 76 is fixed for rotation withcarrier 60 of center differential assembly 32. A drive gear 78 issupported for rotation on second front axle shaft 28. A driven gear 80is in meshed engagement with drive gear 78 and fixed for rotation with aring gear 82 of right-angled drive assembly 36. Driven gear 80 and ringgear 82 are fixed for rotation with a countershaft 84. Synchronizerclutch 34 selectively drivingly interconnects input shaft 76 and drivegear 78. Synchronizer clutch 34 includes a hub 86 fixed for rotationwith input shaft 76. An axially moveable sleeve 88 is in splinedengagement with hub 86. A second hub 90 is fixed for rotation with drivegear 78 and includes an external spline 92. Synchronizer clutch 34 alsoincludes a blocker ring 94 positioned between hub 86 and second hub 90.Blocker ring 94 functions to assure that the rotational speed of inputshaft 76 is substantially the same as drive gear 78 prior to allowing adriving connection between hub 86 and second hub 90 via sleeve 88. Itshould be appreciated that an alternate synchronizer (not shown) may notrequire a blocker ring to function properly.

A synchronizer clutch actuation mechanism 96 includes a shift fork 98slidingly positioned with a groove 100 formed in sleeve 88. An actuator102 is operable to move fork 98 and sleeve 88 from a first positionwhere sleeve 88 is disengaged from spline 92 and a second position wheresleeve 88 concurrently drivingly engages hub 86 and second hub 90.

Right-angled drive assembly 36 includes ring gear 82 and a pinion gear108 in meshed engagement with ring gear 82. Pinion gear 108 may beintegrally formed with a pinion shaft 110. Pinion shaft 110 is fixed forrotation with propeller shaft 38 via a flange 112. Synchronizer clutch34 may be placed in an activated mode where torque is transferredbetween input shaft 76 and drive gear 78. Synchronizer clutch 34 is alsooperable in a deactivated mode where no torque is transferred to reardriveline 14. Power from engine 16 is not transferred to right-angleddrive assembly 36 when synchronizer clutch 34 is in the deactivatedmode.

Friction coupling 39 is depicted as a friction clutch fixed to a rearaxle assembly 113. Rear axle assembly 113 includes differential assembly50, rear axle shaft 44, rear axle shaft 46 and disconnect 52.Differential assembly 50 includes a carrier housing 114 fixed forrotation with a ring gear 115 of ring and pinion gearset 48.Differential assembly 50 also includes first and second side gears 116,117 fixed for rotation with first and second rear axle shafts 44, 46,respectively. A pair of pinion gears 118 are positioned within carrierhousing 114 and placed in constant meshed engagement with side gears116, 117. Friction coupling 39 includes a drum 120 fixed for rotationwith propeller shaft 38. A hub 122 is fixed for rotation with a pinionshaft 124. A pinion gear 126 of pinion gearset 48 may be integrallyformed with pinion shaft 124. Outer clutch plates 128 are splined forrotation with drum 120. A plurality of inner clutch plates 130 aresplined for rotation with hub 122 and interleaved with outer clutchplates 128. An actuator 134 is operable to apply a clutch actuationforce to clutch plates 128, 130 and transfer torque through frictioncoupling 39. In one example, an axially moveable piston may be inreceipt of pressurized fluid to provide the actuation force.Alternatively, an electric motor may cooperate with a forcemultiplication mechanism. In yet another embodiment described below ingreater detail, the friction clutch may be actuated based on wheel slipor a difference in rotational speed across the friction clutch.

Disconnect 52 is depicted in FIGS. 1 and 3 as a dog clutch. Disconnect52 includes a first hub 140 fixed for rotation with a shaft 142drivingly engaged with side gear 117 of differential assembly 50. Anexternal spline 146 is formed on first hub 140. An axially translatablesleeve 148 is in splined engagement with first hub 140. A second hub 150is fixed for rotation with rear axle shaft 46. A spline 152 is formed onan outer periphery of second hub 150.

A dog clutch actuation system 156 includes a fork 158 slidablypositioned within a groove 160 formed in sleeve 148. An actuator 162 isoperable to translate fork 158 and sleeve 148 between a first positionwhere sleeve 148 is engaged only with first hub 140 and a secondposition where sleeve 148 simultaneously engages splines 146 and 152 todrivingly interconnect shaft 142 with rear axle shaft 46.

FIG. 3A depicts an alternate rear driveline 14A and rear axle assembly40A. Rear axle assembly 40A is substantially similar to rear axleassembly 40, previously described. Accordingly, like elements willretain their previously introduced reference numerals. Rear axleassembly 40A includes another disconnect identified as disconnect 52A.The elements of disconnect 52A are identified in similar fashion to thecomponents of disconnect 52 except that the suffix “A” has been added.Disconnect 52A may selectively drivingly connect and disconnect rearaxle shaft 44 with an axle shaft portion 142A that is fixed for rotationwith side gear 116. During operation, ring and pinion gearset 48 anddifferential assembly 50 may be entirely disconnected from rear axleshaft 44 and rear axle shaft 46. Accordingly, even the internalcomponents of differential assembly 50 will not be rotated due to inputfrom rear wheels 42. To return to the all wheel drive mode of operation,actuator 162A is controlled at substantially the same time as actuator162 to reconnect shaft 142A and rear axle shaft 44 in the same manner asshaft 142 is coupled to rear axle shaft 46.

FIGS. 4 and 5 depict an alternate drive train 10′. Drive train 10′ issubstantially similar to drive train 10. As such, like elements will beidentified with the previously introduced reference numerals including aprime suffix. Drive train 10′ includes a power take-off unit 20′ thatdiffers from power take-off unit 20 by being a single axis powertransmission device that does not include countershaft 84, previouslydescribed. On the contrary, power take-off unit 20′ includes aconcentric shaft 166 having ring gear 82′ fixed thereto. Ring gear 82′is in meshed engagement with pinion gear 108′ to drive propeller shaft38′.

FIGS. 4 and 5 also show that disconnect 52 may be alternatively formedas a roller clutch identified as reference numeral 52′. The drivelinedepicted in FIGS. 1 and 4 may include either a dog clutch, a rollerclutch or one of a number of other power transmission devices operableto selectively transfer torque and cease the transfer of torque betweenrotary shafts. In the example depicted, roller clutch 52′ includes aninner member 170 fixed for rotation with rear axle shaft 46′ and anouter member 172 fixed for rotation with shaft 142′. Inner member 170includes a surface 174 having a plurality of curved recesses. Eachrecess is in receipt of a roller 176. A split ring 178 is positionedbetween rollers 176 and outer member 172. Split ring 178 also includes aplurality of curved recesses facing the recesses of inner member 170 andin receipt of rollers 176. A control arm 180 cooperates with split ring178 to restrict or permit relative rotation between inner member 170 andsplit ring 178. When relative rotation is permitted, rollers 176 areforced radially outwardly to radially outwardly expand split ring 178into engagement with outer member 172 to transfer torque across rollerclutch 52′. When relative rotation between inner member 170 and splitring 178 is restricted, rollers 176 are not displaced, the rollers arenot wedged between split ring 178 and inner member 170 and torque is nottransferred across disconnect 52′. An actuator 182 may move control arm180 to operate disconnect 52′.

During vehicle operation, it may be advantageous to reduce the churninglosses associated with driving ring and pinion gearset 48 as well asright-angled drive assembly 36. With reference to FIG. 1, a controller190 is in communication with a variety of vehicle sensors 192 providingdata indicative of parameters such as vehicle speed, four-wheel drivemode, wheel slip, vehicle acceleration and the like. One sensor 192 maybe positioned at a location proximate ring and pinion gearset 48 toprovide a signal indicating the rotational speed of a ring and piniongearset component. At the appropriate time, controller 190 may output asignal to control actuator 96 and place synchronizer clutch 34 in thedeactuated mode where torque is not transferred from engine 16 to reardriveline 14. Controller 190 may also signal actuator 162, associatedwith disconnect 52, to place fork 158 in a position to cease torquetransfer across disconnect 52 such that the energy associated with oneof rotating rear wheels 42 will not be transferred to ring and piniongearset 48 or differential assembly 50. Accordingly, the hypoid gearsets36, 48 will not be driven by differential assembly 32. Furthermore,because side gear 116 is not restricted from rotation, input torqueprovided by rear axle shaft 44 will only cause the internal gears withindifferential assembly 50 to rotate. Ring and pinion gearset 48 is notdriven. It should be appreciated that friction coupling 39 may beoperated in either of an open mode or a torque transferring mode whensynchronizer clutch 34 and disconnect 52 do not transfer torque becauserear driveline 14 is not rotating at this time.

When controller 190 determines that a four wheel drive mode of operationis to commence, controller 190 signals actuator 102 to slide sleeve 88toward hub 90. During this operation, speed synchronization betweeninput shaft 76 and drive gear 78 occurs. Once the speeds are matched,sleeve 88 drivingly interconnects hub 86 and second hub 90. At thistime, right-angled drive assembly 36 is also driven by engine 16. Oncethe front driveline components and the right-angled drive components areup to speed, controller 190 provides a signal to actuator 134 to beginspeed synchronization of ring and pinion gearset 48 as well asdifferential assembly 50. This sequence of operations will cause thespeed of shaft 142 to match the speed of rear axle shaft 46. At thistime, controller 190 provides a signal to actuator 162 to placedisconnect 52 in a torque transferring mode by axially translatingsleeve 148. At the end of this sequence, drive train 10 is operable inan all wheel drive mode. It should be appreciated that the procedurepreviously described may be performed while the vehicle is moving.

It is contemplated that friction coupling 39 may be alternativelyconfigured as a passive device having an actuation system operable inresponse to a speed differential between propeller shaft 38 and pinionshaft 124. In particular, FIG. 4 depicts friction coupling 39′ includinga pump 202 driven by propeller shaft 38 when a speed differential existsbetween propeller shaft 38′ and pinion shaft 124. Pressurized fluid frompump 202 is provided to a piston 204 for applying a compressive force toinner clutch plates 130′ and outer clutch plates 128′. In thisarrangement, control of synchronizer clutch 34′ also provides control offriction coupling 39′ because rotation of propeller shaft 38′ relativeto pinion shaft 124′ will cause pressurized fluid to cause torquetransfer across friction coupling 39′ thereby quickly achieving speedsynchronization of the front driveline and rear driveline components.Furthermore, the inclusion of friction coupling 39 or 39′ allowssynchronizer clutch 34 or 34′ to be relatively minimally sized becauseonly some of the components of power transmission device 20 andpropeller shaft 38 are speed synchronized through actuation ofsynchronizer clutch 34. The relatively large rotating masses within rearaxle assembly 40 are accelerated through actuation of friction coupling39.

While a number of vehicle drivelines have been previously described, itshould be appreciated that the particular configurations discussed aremerely exemplary. As such, it is contemplated that other combinations ofthe components shown in the Figures may be arranged with one another toconstruct a drive train not explicitly shown but within the scope of thepresent disclosure.

What is claimed is:
 1. A drivetrain for a motor vehicle having a powersource and first and second sets of wheels, the drivetrain comprising: afirst driveline adapted to normally transfer drive torque from the powersource to the first set of wheels, the first driveline including a firstdifferential assembly having a first differential input component drivenby a drive gear of the power source and a pair of first differentialoutput components interconnected to the first set of wheels; a seconddriveline adapted to selectively transfer drive torque from the powersource to the second set of wheels, the second driveline including afirst hypoid gearset, a first disconnect coupling operable forselectively connecting and disconnecting the first hypoid gearset to oneof the drive gear of the power source and the first differential inputcomponent of the first differential assembly, a propshaft driven by thefirst hypoid gearset, a second differential assembly having a seconddifferential input component driven by a second hypoid gearset and apair of second differential output components interconnected to thesecond set of wheels, a torque coupling for selectively connecting adisconnecting the propshaft to the second hypoid gearset, and a seconddisconnect coupling for selectively connecting and disconnecting one ofthe second differential output components of the second differentialassembly to its corresponding one of the second set of wheels; and acontrol system for controlling actuation of the first disconnectcoupling, the torque coupling and the second disconnect coupling forselectively switching the vehicle between a two-wheel drive mode and anall-wheel drive mode.
 2. The drivetrain of claim 1 wherein the controlsystem is operable to shift the vehicle from the two-wheel drive modeinto the all-wheel drive mode by sequentially actuating the firstdisconnect coupling to synchronize the rotary speed of the first hypoidgearset with the rotary speed of the drive gear, actuating the torquecoupling to synchronize the rotary speed of the second hypoid gearsetwith the rotary speed of the first hypoid gearset, and actuating thesecond disconnect coupling to connect the one of the second differentialoutput components of the second differential assembly to thecorresponding one of the second set of wheels.
 3. The drivetrain ofclaim 2 wherein the first disconnect coupling connects the drive gear tothe first hypoid gearset prior to actuation of the torque coupling,wherein the torque coupling can be released after the second disconnectcoupling connects the one of the second differential output componentsof the second differential assembly to the corresponding one of thesecond set of wheels, and wherein the actuated state of the torquecoupling can be subsequently modulated following the initial release tovary the magnitude of drive torque transferred from the power source tothe second set of wheels.
 4. The drivetrain of claim 1 wherein thesecond driveline includes a power take-off unit, wherein the firstdisconnect coupling and the first hypoid gearset are positioned withinthe power take-off unit, and wherein the first disconnect coupling is asynchronizer clutch having a clutch sleeve axially moveable between afirst position and a second position, the clutch sleeve being operablein its first position to disconnect an input member of the first hypoidgearset from a drive shaft connected to at least one of the drive gearand the first differential input component of the first differentialassembly, and the clutch sleeve being operable in its second position todrivingly connect the input member of the first hypoid gearset to thedrive shaft.
 5. The drivetrain of claim 4 wherein the first hypoidgearset includes a first ring gear that is meshed with a first piniongear drivingly connected to a first end of the propshaft, wherein theinput member of the first hypoid gearset includes a transfer gearsethaving a first transfer gear fixed for rotation with the first ring gearand a second transfer gear meshed with the first transfer gear, andwherein the clutch sleeve is operable in its first position to releasethe second transfer gear from driven connection with the drive shaft andis operable in its second position to drivingly couple the secondtransfer gear to the drive shaft.
 6. The drivetrain of claim 1 whereinthe second driveline further includes a third disconnect couplingoperable for selectively connecting and disconnecting the other one ofthe second differential output components of the second differentialassembly to the other one of the second set of wheels, and wherein thecontrol system is operable to control actuation of the third disconnectcoupling.
 7. The drivetrain of claim 1 wherein the torque coupling is amultiplate friction clutch having a first clutch member fixed forrotation with the propshaft, a second clutch member fixed for rotationwith an input component of the second hypoid gearset, a clutch packdisposed between the first and second clutch members, and an actuatoroperable to exert a clutch engagement force on the clutch pack.
 8. Thedrivetrain of claim 1 wherein the second disconnect coupling is a dogclutch having a clutch sleeve moveable between first and secondpositions, the clutch sleeve is operable in its first position torelease the one of the second differential output components of thesecond differential assembly from driven engagement with an axle shaftdrivingly connected to the one of the second pair of wheels, the clutchsleeve is operable in its second position to couple the one of thesecond differential output components of the second differentialassembly for driven rotation with the axle shaft.
 9. A drivetrain for amotor vehicle having a power service and first and second sets ofwheels, the drivetrain comprising: a first driveline adapted to normallytransfer drive torque from the power source to the first set of wheels,the first driveline including a drive gear for transferring drive torquefrom the power source to a first differential assembly operablyinterconnected to the first set of wheels; a second driveline adapted toselectively transfer drive torque from the power source to the secondset of wheels, the second driveline including a first hypoid gearset, afirst disconnect coupling operable for selectively connecting anddisconnecting the first hypoid gearset with the drive gear, a propshaftdriven by the first hypoid gearset, a second hypoid gearset driving asecond differential assembly operably interconnected to the second setof wheels, a torque coupling operable for transferring drive torque fromthe propshaft to the second hypoid gear, and a second disconnectcoupling operable for selectively connecting and disconnecting thesecond set of wheels with the second hypoid gearset; and a controlsystem for controlling actuation of at least the first and seconddisconnect couplings to selectively switch the vehicle between atwo-wheel drive mode and an all-wheel drive mode, wherein the first andsecond hypoid gearsets and the propshaft are disconnected from the powersource and the second set of wheels when the first and second disconnectcouplings are operated in disconnected, non-torque transferring modes toestablish the two-wheel drive mode, and wherein the vehicle is shiftedfrom the two-wheel drive mode into the all-wheel drive mode by actuatingthe first disconnect coupling and subsequently actuating the seconddisconnect clutch.
 10. The drivetrain of claim 9 wherein the controlsystem is further operable to control actuation of the torque coupling,and wherein the torque coupling is actuated prior to actuation of thesecond disconnect coupling during a switch from the two-wheel drive modeinto the all-wheel drive mode.
 11. The drivetrain of claim 10 whereinthe control system is operable to shift the vehicle from the two-wheeldrive mode into the all-wheel drive mode by sequentially actuating thefirst disconnect coupling to synchronize the rotary speed of the firsthypoid gearset with the rotary speed of the drive gear, actuating thetorque coupling to synchronize the rotary speed of the second hypoidgearset with the rotary speed of the first hypoid gearset, and actuatingthe second disconnect coupling to connect the second differentialassembly to the second set of wheels.
 12. The drivetrain of claim 11wherein the first disconnect coupling connects the drive gear to thefirst hypoid gearset prior to actuation of the torque coupling, whereinthe torque coupling can be released after the second disconnect couplingconnects the second differential assembly to the second set of wheels,and wherein the actuated state of the torque coupling can besubsequently modulated following the initial release to vary themagnitude of drive torque transferred from the power source to thesecond set of wheels.
 13. The drivetrain of claim 9 wherein the seconddriveline includes a power take-off unit, wherein the first disconnectcoupling and the first hypoid gearset are positioned within the powertake-off unit, and wherein the first disconnect coupling is asynchronizer clutch having a clutch sleeve axially moveable between afirst position and a second position, the clutch sleeve being operablein its first position to disconnect an input member of the first hypoidgearset from a drive shaft connected to at least one of the drive gearand a differential input component of the first differential assembly,and the clutch sleeve being operable in its second position to drivinglyconnect the input member of the first hypoid gearset to the drive shaft.14. The drivetrain of claim 9 wherein the second driveline furtherincludes a third disconnect coupling operable for selectively connectingand disconnecting the second differential assembly to the second set ofwheels, and wherein the control system is operable to control actuationof the third disconnect coupling.
 15. A vehicle drive train fortransferring torque from a power source to first and second sets ofwheels, the drive train comprising: a first driveline adapted totransfer torque from the power source to the first set of wheels andincluding a differential assembly, first hypoid gearset and asynchronized first disconnect clutch positioned between the differentialassembly and the first hypoid gearset to selectively transfer or ceasethe transfer of drive torque from the power source to the first hypoidgearset; and a second driveline having a second hypoid gearset inreceipt of drive torque from the first hypoid gearset, the seconddriveline transferring drive torque from the second hypoid gearset tothe second set of wheels and including a non-synchronized seconddisconnect clutch selectively interrupting the transfer of drive torquefrom the second set of wheels to the second hypoid gearset, the seconddriveline also including a friction clutch for transferring drive torquebetween the first hypoid gearset and the second hypoid gearset.
 16. Thevehicle drive train of claim 15 wherein the second driveline includes adrive axle including the second hypoid gearset, a second differentialassembly, axle shafts and the non-synchronized second disconnect clutch,the second differential assembly including a carrier containing a pairof pinion gears in meshed engagement with a pair of side gears, thenon-synchronized second disconnect clutch being positioned between oneof the side gears and one of the axle shafts.
 17. The vehicle drivetrain of claim 15 which is switchable from a two-wheel drive mode to afour-wheel drive mode by transferring drive torque first through thesynchronized first disconnect clutch, next through the friction clutchand lastly through the non-synchronized second power disconnect clutch.18. A method for transferring torque from a power source to a first pairand a second pair of wheels in a vehicle drivetrain, the methodcomprising: transferring torque from the power source to the first pairof wheels; actuating a first disconnect clutch to initially synchronizerotary speeds between and subsequently transfer torque from the powersource to a driveline unit; actuating a friction clutch to transfertorque from the driveline unit to a drive axle to initiate rotation of agearset within the drive axle; and actuating a second disconnect clutchto drivingly interconnect a shaft coupled to one wheel of the secondpair of wheels and a rotatable member of the drive axle once speedsynchronization between the components coupled by the first disconnectclutch is achieved.
 19. A drivetrain for a motor vehicle having a powersource and first and second set of wheels, the drivetrain comprising: afirst driveline adapted to transfer torque from the power source to thefirst set of wheels, the first driveline including a first differentialassembly and a power take-off unit, the first differential assemblyoperably interconnecting the power source to the first set of wheels,the power take-off unit including a first hypoid gearset and a firstdisconnect coupling operable for selectively connecting anddisconnecting the first hypoid gearset to at least one of the powersource and the first differential assembly; a second driveline adaptedto selectively transfer torque from the first hypoid gearset to thesecond set of wheels, the second driveline including a propshaft drivenby the first hypoid gearset, a second differential assembly operablyinterconnected to the second set of wheels, a second hypoid gearsetdriving the second differential assembly, a torque coupling operable forselectively connecting and disconnecting the propshaft to the secondhypoid gearset, and a second disconnect coupling operable forselectively connecting and disconnecting one of the second set of wheelsto the second differential assembly; and a control system forcontrolling actuation of the first disconnect coupling, the torquecoupling and the second disconnect coupling for selectively switchingthe vehicle between a two-wheel drive mode and a four-wheel drive mode.20. The drivetrain of claim 19 wherein the control system is operable toshift the vehicle from the two-wheel drive mode into the four-wheeldrive mode by sequentially actuating the first disconnect coupling totransfer torque from the power source to the propshaft through the firsthypoid gearset, the torque coupling to transfer torque from thepropshaft to the second differential assembly, and the second disconnectcoupling to transfer torque from the second differential assembly to thesecond set of wheels.
 22. A drivetrain for a vehicle, comprising: apower source having a rotary output component; a set of front wheels; afront drive assembly including a drive gear driven by the rotary outputcomponent of the power source and a front differential assembly having afront differential input driven by the drive gear and a pair of frontdifferential outputs driving the set of front wheels; a set of rearwheels; and a rear drive assembly switchable between a two-wheel drive(2WD) mode and an all-wheel drive AWD mode, wherein the rear driveassembly is operating in its 2WD mode when the power source onlytransmits drive torque to the front wheels, wherein the rear driveassembly is operating in its AWD mode when the power source transmitsdrive torque to the front wheels and the rear wheels, the rear driveassembly including a first hypoid gearset, a second hypoid gearset, apropshaft interconnecting an output of the first hypoid gearset to aninput of the second hypoid gearset, a rear differential assembly havinga rear differential input driven by an output of the second hypoidgearset and first and second rear differential outputs, a firstdisconnect clutch disposed between the drive gear and an input to thefirst hypoid gearset, a second disconnect clutch disposed between thefirst rear differential output and one of the rear wheels, and a thirddisconnect clutch disposed between the second rear different output andthe other one of the rear wheels; and a control system for controllingactuation of the first, second and third disconnect clutches to switchthe rear drive assembly between the 2WD and AWD modes, wherein the 2WDmode is established when the first, second and third disconnect clutchesare each in a disconnected, non-torque transferring state, and whereinthe AWD mode is established by initially shifting the first disconnectclutch into a connected torque-transferring state while maintaining saidsecond and third disconnect clutches in their disconnected, non-torquetransferring states for synchronizing the rotary speed of the firsthypoid gearset with the rotary speed of the drive gear and subsequentlyshifting the second and third disconnect clutches into their connectedtorque-transferring states.
 23. The drivetrain of claim 22 wherein saidrear drive assembly further includes a torque coupling disposed betweenthe propshaft and the input to the second hypoid gearset, wherein saidcontrol system control actuation of said torque coupling, and whereinsaid torque coupling is shifted from a disconnected non-torquetransferring state into a connected torque-transferring state followingactuation of said first disconnect clutch and prior to actuation of saidsecond and third disconnect clutches.
 24. The drivetrain of claim 22wherein said first disconnect clutch is a synchronizer clutch having aclutch sleeve moveable between first and second positions, the clutchsleeve is operable in its first position to disconnect the input of thefirst hypoid gearset from driven connection with the drive gear, and theclutch sleeve is operable in its second position to connect the input ofthe first hypoid gearset for driven connection with the drive gear.